The proposed study is a continuation of our previous research program. The objective is to determine the extent to which structural elements of viral nucleic acids and proteins may direct their biological functions. We hope that this will contribute to a full understanding of the replicative cycle of two families of animal RNA viruses: Picornaviruses and RNA tumor viruses (retroviruses). We propose, firstly, to analyze the structure of RNAs and proteins of poliovirus on three different levels: (1) a study of the secondary structure of poliovirus RNAs by electron microscopy; (2) the determination of the complete nucleotide sequence of the genome of poliovirus, type 1 (Mahoney); and (3) the determination of the complete amino acid sequence of 3 viral proteins: VP4, PV2 and VPg, and an analysis of the C-terminal amino acid and several (4-6) NH2-terminal amino acids of a number of virus specific proteins. We shall investigate the properties and function of the genome-linked protein "VPg" of poliovirus; and we propose to analyze the mechanism of poliovirus protein synthesis and of poliovirus RNA replication. We believe that the study will yield solutions to a variety of unsolved problems in the molecular biology of picornaviruses. An important result may be an insight in mechanisms involved in processing of proteins not only of picornarivuses but also of RNA tumor viruses and other viruses, and their host cells. Secondly, we propose to analyze sequences of retrovirus RNAs, specifically of murine leukemia and murine sarcoma viruses. We shall attempt to characterize 3'-terminal sequence of MLV and MSV. These segments of the two retrovirus genomes may contain homologous sequences which are possibly involved in the replication, and heterologous sequences belonging to separate genes, e.g., the env gene and the sarc gene, respectively.